"I've been here so many times and finally I did it," Radwanska said.
There were eight straight service breaks in the first set, which Li lost with her eighth double-fault. Pennette broke Li twice in the second set, and she closed out the match that featured several long rallies with a backhand winner down the line.
"In the beginning of the week if you ask me, 'What do you think, you going to make a final here?' I would say, `Mmmm, I don't think so," Pennetta said. "But my tennis, it's good. I am pretty happy in the way I play."
Li, the Australian Open champion, fell to 17-2 on the year, plagued by more than 50 unforced errors. She was playing her first major tournament as a No. 1 seed.
"It was pretty high-level match," Li said. "I lose the match, a little bit sad, but I still can see a lot positive. At least, I like the way I hit the ball."
In the other semifinal, Halep had Radwanska on the run most of the match, but Radwanska was up to the challenge, continually chasing down balls and making some well-placed returns. Radwanska was unable to convert on her first two match points on Halep's serve in the ninth game. Radwanska's swinging forehand volley set up her third in the next game and she won on Halep's backhand that went wide.
"What I was trying to do was playing aggressive from the beginning of the match and just trying to go for my shots," Radwanska said. "Pretty much all the games, doesn't matter who was serving, it was really tight games. I was better just a few points each set."
Halep, who had 21 winners, is projected to move up two spots to a career-best No. 5 in the world in Monday's rankings, the highest ever by a Romanian woman.